1. Field of the Invention
The present invention relates to a mobile communication system, and a base station device and a mobile wireless terminal used in the same system, and more particularly to the mobile communication system, the base station device, and the mobile wireless terminal which make data transmission by using an HSDPA (High Speed Downlink Packet Access) method.
The present application claims priority of Japanese Patent Application No. 2003-413697 filed on Dec. 11, 2003, which is hereby incorporated by reference.
2. Description of the Related Art
In a W-CDMA (Wideband Code Division Multiple Access) communication system in conformity with 3GPP (3rd Generation Partnership Project) being now standardized, the HSDPA is proposed as technology that can realize a high-speed downlink.
In the HSDPA method, as a physical channel for a downlink through which data are transmitted from a base station to a mobile wireless terminal, an HS-SCCH (High Speed—Shared Control CHannel) and an HS-PDSCH (High Speed-Physical Downlink Shared CHannel) are additionally provided. The HS-SCCH is used to transmit control information of the HS-PDSCH pairing up with the HS-SCCH, while the HS-PDSCH is used to transmit packet data employed in the HSDPA method. Moreover, the HS-PDSCH can use a maximum 15 pieces of channelization codes in units of subframes, which is called a multi-code, and the number of used multicodes is called the multicode number. The subframe denotes a frame length of a physical channel used in the HSDPA method expressed in time units of three slots and a slot denotes a time unit designated by the 3GPP.
On the other hand, as a physical channel for an uplink through which the mobile wireless terminal transmits packet data to the base station, an HS-DPCCH (High Speed-Dedicated Physical Control CHannel) is additionally provided. In the communication using the HSDPA method, two channels including the HS-DPCCH and a DPCH (Dedicated Physical CHannel) are used when packet data is transmitted from the mobile wireless terminal to the base station. The HS-DPCCH informs the base station of whether or not packet data transmitted through the HS-PDSCH pairing up with the HS-DPCCH has been received by the mobile wireless terminal, or of a state of a transmission path. FIG. 9 shows a time relationship of data transmission among the HS-SCCH, HS-PDSCH, and HS-DPCCH.
In the present W-CDMA communication system, whether each TFC (Transport Format Combination) of an uplink DPCH is usable or not is determined by using information about a past estimated transmission power value of the mobile wireless terminal. One of standards for determining the use of a given TFC is whether or not the TFC is one that was allowed to be used based on the past estimated transmission power value of the mobile wireless terminal. Here, the TFC denotes a plural transport channel combining pattern of a TF (Transport Format) showing an information size of each transport channel. Moreover, the estimated transmission power value represents a power value assumed as necessary for the mobile wireless terminal to transmit data to the base station. The reason why the use of the TFC of the DPCH is determined by using an estimated transmission power value is that, in general, power to be supplied has to be increased in proportion to an information size of data to be transmitted. Therefore, in a case where the past estimated transmission value frequently exceeds maximum transmission power, processing is performed in which power to be supplied is reduced by decreasing a size of information to be transmitted through an uplink DPCH.
According to 3GPP TS25.321 v5.5.0 or TS25.133 v5.7.0 in which specifications and procedures of the 3GPP are specified, whether or not each TFC of the uplink DPCH is usable is determined by using an estimated transmission value of the mobile wireless terminal during a specified period and it takes several tens of milliseconds or more before data reflecting this decision are actually transmitted.
As one of technologies using such the HSDPA method as above, a mobile communication system, base station, packet transmitting timing controlling method and program for the same are disclosed in Japanese Patent Application Laid-open No. 2002-369235. In the technology disclosed in the Japanese Patent Application Laid-open No. 2002-369235, the base station, when receiving data to be transmitted to a mobile station from a base station control device, feeds an advance notice (notification of transmitting timing) of data transmission to the mobile station. The mobile station, when receiving the advance notice of data transmission, notifies another base station being in a state of soft handover of timing of the data transmission. This prevents the above other base station from transmitting HS-PDSCH data to other mobile stations.
Also, a mobile communication system, communication control method, base station to be used for the above, and mobile station are disclosed in Japanese Patent Application Laid-open No. 2003-199173 in which, the base station, when receiving data to be transmitted to the mobile station, feeds an advance notice of data transmission to the mobile station by using a downlink DPCH. The mobile station, when receiving the above advance notice from the base station, transmits information about transmission quality to the base station.
As described above, in the communication using the HSDPA method, the HS-DPCCH is added to an uplink physical channel. In a case where an estimated transmission power value exceeds an allowable maximum transmission power value due to this addition of the HS-DPCCH, in an actual transmission, control is exercised so that power being assigned to both the HS-DPCCH and uplink DPCH is reduced so that the expected transmission power value does not exceed the maximum transmission power finally. As a result, it is predicted that, since power to be assigned to the uplink DPCH becomes small, a possibility that the TFC of the DPCH being unusable increases.
However, conventionally, it is impossible to judge whether the expected transmission power value exceeds the maximum transmission power due to the addition of the HS-DPCCH unless packet data was actually transmitted, by using the HS-DPCCH, from the mobile wireless terminal. Furthermore, in the conventional technology, even if the TFC of the uplink DPCH is judged to be unusable according to the above judging method, it takes several tens of milliseconds or more before data having reflected the judgement result are actually transmitted and, therefore, whether the TFC is usable or unusable cannot be immediately reflected in the DPCH. There is a high possibility that packet data transmitted through the HS-PDSCH of the downlink employed in the HSDPA method has occurred in a manner of a burst during a period in which the TFC cannot be made unusable, and a possibility increases that, in the uplink, data is frequently transmitted through the HS-DPCCH due to occurrence of the packet data transmitted through the HS-PDSCH. That is, at the time of transmission of data using the HS-DPCCH in which reduction of the power to be assigned to the uplink DPCH is required, there is a risk that an operation is performed with the power to be assigned to the DPCH being not reduced. This operation leads to a failure in the detection of transmitted data by the uplink DPCH at the base station, since, for a period of time of several tens of milliseconds or more to be taken after transmission of data to be transmitted through the uplink HS-DPCCH in response to data transmitted through the downlink HS-PDSCH through which a head of packet data is transmitted in the burst manner has been started before information that the TFC of the downlink HS-PDSCH cannot be used is actually reflected on the uplink PDCH, power being smaller than expected is assigned to both the uplink DPCH and the downlink HS-DPCCH.
There is a possibility that the conventional technology presents a problem in that the failure in the detection of transmitted data through the uplink DPCH causes a decrease in the communication quality and the failure in the detection of tranmitted data through the HS-DPCCH causes re-transmission of data through downlink HS-PDSCH pairing with the uplink DPCH which leads to a remarkable decrease in throughput of the downlink.
Thus, when the HSDPA method is applied to the conventional W-CDMA communication system constructed in conformity with the 3GPP, there may be a case in which the estimated transmission power exceeds the allowable maximum transmission power due to the addition of the HS-DPCCH and, therefore, the number of TFCs of the uplink DPCH that becomes unusable increases. However, the above conventional technology has the following problem, that is, since it took several tens of milliseconds or more before the information that the TFC of the downlink HS-PDSCH cannot be used is actually reflected on the uplink DPCH after transmission of data to be transmitted through the uplink HS-DPCCH in response to the data transmitted through the downlink HS-PDSCH through which a head of packet data has been transmitted in the burst manner has been started, it causes decreases in throughput of the downlink and in communication quality of the uplink.
Moreover, since, in the technology disclosed in the Japanese Patent Application Laid-open No. 2002-369235 and Japanese Patent Application Laid-open No. 2003-199173, configurations and functions to suppress a decrease in the communication quality caused by a shortage of power to be assigned for data transmission by such the HS-DPCCH as above are not provided and, as a result, it is difficult to solve the above problems.